The Mississippian Barnett Shale is an important hydrocarbon source rock and has recently developed into a very active gas-shale play. Commonly viewed as a homogeneous black shale, the Barnett actually consists of a variety of organic-rich lithofacies of siliceous, calcareous, or phosphatic composition. Recognition of the different lithofacies is an important step in the evaluation of gas in place, flow capacity, and mechanical properties of the Barnett. Petrographic study of conventional core samples from the lower part of the Barnett has led to the recognition of the following rock types: organic-rich black shale, fossiliferous shale, dolomite rhomb shale, dolomitic shale, phosphatic shale, and concretionary carbonate. Measured total organic carbon values, averaged by rock type, range by a factor of nearly two, with the organic-rich shale and phosphatic shale having the highest values. The development of these lithofacies is a result of suspension fallout and sediment gravity flows combined with the extensive early microbial alteration of abundant organic matter. The lithofacies vary in petrophysical and mechanical properties, as well as organic content.